Putting that to the side both the database and the basic FMGC software is routinely updated, any one of those updates can introduce malicious code with expanded functionality.
In the same way that the many computers in modern aircraft are routinely updated.
Most (I cannot say all as I do not know them all) aircraft avionics software updates are done through hard wired data loaders not wifi. The hackers would need to break into the software manufacturers and alter the software from the ground up. This idea of hacking into aircraft systems is fanciful. Even if the IFE or Satcom could be hacked remotely, there are a couple of big buttons on the flight deck that will cut the power to them instantly if requred.